Reflection seismology is a widely employed tool for the exploration of hydrocarbons. A ramification in this technology consists of using horizontally polarized shear waves (SH waves) in addition to more conventional exploration involving compressional (P) waves. In principle, the source for SH wave exploration must produce only SH waves and the earth must be strictly laterally homogeneous (Garotta, 1985). Under these assumptions, the horizontally polarized receivers are oriented along the y-direction perpendicular to the seismic line (x-direction). Any departure from either the source or the earth model, would of course cause recording of undesired modes of waves (designated “noise”) by the receivers.
In particular, most SH sources also generate P waves, while many dynamite sources (e.g. in ±y method: Waters, 1987) generate P waves in the y (transverse) direction too (Edelman, 1985). A relevant result from Aki & Richards (1980) is that for a point source comprising of either a single force or a double couple in a homogeneous medium, the intermediate field displacements, both P and S, involve both radial and transverse components as does the near field displacement. Thus, a typical SH source would contain P motion even along y direction. The foregoing is based on the linear elastic theory, though a nonlinear physics governing the near-source region is expected to engender further complications not envisaged in the linear theory. The receiver, therefore, would record, in addition to the desired SH reflections; reflected P, scattered P-P and P-SH, all having components of motion along y direction. While the first (reflected P) owes its origin to the y motion of P in the source itself, the last two result from an interaction of P waves with lateral heterogeneity, which, though present, is not incorporated in the simplified model of the earth. The lateral heterogeneity can also give rise to SH-P type of scattering with components of motion along y direction. Therefore, due to non-ideal nature of the source and the earth, non-SH types of waves masquerade as SH-reflections and cause substantial error in processing and interpretation of data. Conventional recording methods can not circumvent this problem.